Abstracts Category : Other

This Is Just the Beginning: The Hunt for AstrophysicalNeutrinos

by Weishi Li

Astrophysical neutrinos are precious messengers of ourUniverse. They reveal information about the core of the Sun, theinteriors of exploding stars, the sites of cosmic-ray acceleration,and fundamental properties of neutrinos. Yet, the study ofastrophysical neutrinos has been a difficult quest. Becauseneutrinos interact only weakly with matter, we need kilo-ton-scaleor even larger detectors to detect reasonable numbers of signalevents. Modest numbers of signal events are overwhelmed by largenumbers of background events, i.e., events that look like neutrinosbut they are caused by other processes. On top of that, we arelimited in what neutrino properties we can extract from detectedevents, and with what precision.In this dissertation, I discuss aseries of papers aimed at improving our measurements ofastrophysical neutrinos. Solar neutrinos, with energies around 10MeV, have been detected in Super-Kamiokande for 20 years, yet somekey measurement results are still inconclusive. One limiting factoris the spallation background induced by cosmic-ray muons. To betterreject this background, I first calculate the spallation yields inSuper-Kamiokande. I then study the production mechanisms ofspallation backgrounds in detail and conclude that they depend onmuon-induced electromagnetic and hadronic showers, rather than bymuons themselves. Next, I explore how to reconstruct showers inSuper-Kamiokande with high fidelity and propose a new spallationcut utilizing better-reconstructed shower profiles. On the TeV PeV astrophysical neutrino front, one experimental limitation withcurrent detection techniques is that they cannot distinguishbetween two neutrino types, or flavors, e and _. Tosolve this problem, I propose two new shower-related experimentalobservables, muon echoes and neutron echoes, that are stronger in_-initiated events.The methods presented in thisdissertation have been adopted by neutrino experimentalcollaborations, with results yet to come out. This is just thebeginning.Advisors/Committee Members: Beacom, John (Advisor).